<sect1 id="ai-spiralgal">

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<author><firstname>Mike</firstname> <surname>Choatie</surname> </author>
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<title>Spiral Galaxies</title>
<indexterm><primary>Spiral Galaxies</primary>
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<para>Spiral galaxies are huge collections of billions of stars, most of which are flattened into a disk shape, with a bright, spherical bulge of stars at its centre. Within the disk, there are typically bright arms where the youngest, brightest stars are found. These arms wind out from the centre in a spiral pattern, giving the galaxies their name. Spiral galaxies look a bit like hurricanes, or like water flowing down a drain. They are some of the most beautiful objects in the sky. </para>
<para>Galaxies are classified using a <quote>tuning fork diagram</quote>. The end of the fork classifies <link linkend="ai-ellipgal">elliptical galaxies</link> on a scale from the roundest, which is an E0, to those that appear most flattened, which is rated as E7. The <quote>tines</quote> of the tuning fork are where the two types of spiral galaxies are classified: normal spirals, and <quote>barred</quote> spirals. A barred spiral is one whose nuclear bulge is stretched out into a line, so it literally looks like it has a <quote>bar</quote> of stars in its centre. </para><para>Both types of spiral galaxies are sub-classified according to the prominence of their central <quote>bulge</quote> of stars, their overall surface brightness, and how tightly their spiral arms are wound. These characteristics are related, so that an Sa galaxy has a large central bulge, a high surface brightness, and tightly-wound spiral arms. An Sb galaxy has a smaller bulge, a dimmer disk, and looser arms than an Sa, and so on through Sc and Sd. Barred galaxies use the same classification scheme, indicated by types SBa, SBb, SBc, and SBd. </para><para>There is another class of galaxy called S0, which is morphologically a transitional type between true spirals and ellipticals. Its spiral arms are so tightly wound as to be indistinguishable; S0 galaxies have disks with a uniform brightness. They also have an extremely dominant bulge. </para><para>The Milky Way galaxy, which is home to earth and all of the stars in our sky, is a Spiral Galaxy, and is believed to be a barred spiral. The name <quote>Milky Way</quote> refers to a band of very faint stars in the sky. This band is the result of looking in the plane of our galaxy's disk from our perspective inside it. </para><para>Spiral galaxies are very dynamic entities. They are hotbeds of star formation, and contain many young stars in their disks. Their central bulges tend to be made of older stars, and their diffuse halos are made of the very oldest stars in the Universe. Star formation is active in the disks because that is where the gas and dust are most concentrated; gas and dust are the building blocks of star formation. </para><para>Modern telescopes have revealed that many Spiral galaxies harbour supermassive black holes at their centres, with masses that can exceed that of a billion Suns. Both elliptical and spiral galaxies are known to contain these exotic objects; in fact many astronomers now believe that <emphasis>all</emphasis> large galaxies contain a supermassive black hole in their nucleus. Our own Milky Way is known to harbour a black hole in its core with a mass millions of times bigger than a star's mass. </para>

<tip>
<para>There are many fine examples of spiral galaxies to be found in &kstars;, and many have beautiful images available in their <link linkend="popup-menu">popup menu</link>. You can find them by using the <link linkend="findobjects">Find Object</link> window. Here is a list of some spiral galaxies with nice images available: <itemizedlist>
<listitem><para>M 64, the Black-Eye Galaxy (type Sa)</para></listitem>
<listitem><para>M 31, the Andromeda Galaxy (type Sb)</para></listitem>
<listitem><para>M 81, Bode's Galaxy (type Sb)</para></listitem>
<listitem><para>M 51, the Whirlpool Galaxy (type Sc)</para></listitem>
<listitem><para>NGC 300 (type Sd) [use DSS image link]</para></listitem>
<listitem><para>M 83 (type SBa)</para></listitem>
<listitem><para>NGC 1530 (type SBb)</para></listitem>
<listitem><para>NGC 1073 (type SBc)</para></listitem>
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